545
15.5
Problems
anticipates an upper sediment zone continually
influenced by bioirrigation, whereas the case
shown on the right merely assumes surface
coupling in a specific depth - e.g., by the effect
of a particular worm-hole. The result may be
compared with the measurements conducted by
Glud et al. (1994) and shown in Figure 3.32. Just
as Wang and Van Cappellen (1994) have
described it in one application of the model
STEADYSED1, the situation here is that
bioirrigation conveys nitrate into the sediment,
whereas, at the same time, molecular diffusion
transports nitrate from the sediment back into
the bottom water. This results from the fact that
both processes are coupled to the bottom water
layer in different depths of the sediment. In the
case of diffusion, these are the superficial
nitrate-rich zones, whereas in the case of
bioirrigation, the deeper situated zones are
effective, which contain only small amounts of
nitrate.
Ultimately, all these agreements between the
measured profile and the resultant model are not
really significant since they merely represent a
direct application of the model concept of
bioirrigation, i.e., the pore water input conducted
from the bottom water at a certain depth in the
sediment. Yet, these models are only interesting
inasmuch as the non-diffusive transport between
sediment and bottom water results from them.
These are exemplified for iron and manganese in
Figure 15.19, calculated with a similar model
concept using the program STEADYSED1. It
should be kept in mind that these calculations are
only workable to the point as they comply to a
rather simplified model concept and to a
stationary situation anticipated not only in the
‘Press F9 method’ but in STEADYSED1 as well.
Only by conducting measurements of the material
fluxes in-situ at very dissimilar locations, and
after repeated comparisons with the adequate
models have been made, will the previous model
concepts and the calculated fluxes ever become
verified in the future.
Acknowledgements
This is contribution No 0337 of the Research Center
Ocean Margins (RCOM) which is financed by the
Deutsche Forschungsgemeinschaft (DFG) at Bremen
University, Germany.
15.5 Problems
Problem 1
Table 15.1 shows an analysis of ocean water
made by Nordstrom et al. (1979). A condition of
oversaturation of the minerals calcite (SI = 0.76)
and dolomite (SI = 2.4) at a measured pH value
of 8.22 has been calculated for this analysis
with the model PHREEQC (see also Table 15.3).
What pH is to be expected if we calculate the
saturation state of both minerals in this ocean
water sample with PHREEQC under the condition of log pCO 2 = –3.47 ?
Problem 2
At which pCO 2 would the mineral calcite in
the ocean water sample in Table 15.1 (Nordstrom et al., 1979) be saturated if a pH value of
8.22 had been measured ? How could this pCO 2
which is markedly below the value of the
atmospheric be explained ?
Problem 3
What penetration depth of oxygen would
you expect in a sediment, if the removal of
oxygen can be described in terms of a half-life
of 5 days? Sediments have a temperature of
10°C. Their porosity is at 70 percent. For the
diffusion coefficients use Tables 3.1 and 3.2.
The bottom water shall have an oxygen concentration of 200 µmol l
-1
. Use an oxygen concentration less than 1 µmol l
-1
as “zero”.
Problem 4
A different sulfate concentration gradient is
measured above the sulfate-methane transition
zone than underneath the methane zone. How
could this be explained, despite the fact that
sulfate and methane react with each other in a
ratio of 1:1 ?
15.5
Problems
anticipates an upper sediment zone continually
influenced by bioirrigation, whereas the case
shown on the right merely assumes surface
coupling in a specific depth - e.g., by the effect
of a particular worm-hole. The result may be
compared with the measurements conducted by
Glud et al. (1994) and shown in Figure 3.32. Just
as Wang and Van Cappellen (1994) have
described it in one application of the model
STEADYSED1, the situation here is that
bioirrigation conveys nitrate into the sediment,
whereas, at the same time, molecular diffusion
transports nitrate from the sediment back into
the bottom water. This results from the fact that
both processes are coupled to the bottom water
layer in different depths of the sediment. In the
case of diffusion, these are the superficial
nitrate-rich zones, whereas in the case of
bioirrigation, the deeper situated zones are
effective, which contain only small amounts of
nitrate.
Ultimately, all these agreements between the
measured profile and the resultant model are not
really significant since they merely represent a
direct application of the model concept of
bioirrigation, i.e., the pore water input conducted
from the bottom water at a certain depth in the
sediment. Yet, these models are only interesting
inasmuch as the non-diffusive transport between
sediment and bottom water results from them.
These are exemplified for iron and manganese in
Figure 15.19, calculated with a similar model
concept using the program STEADYSED1. It
should be kept in mind that these calculations are
only workable to the point as they comply to a
rather simplified model concept and to a
stationary situation anticipated not only in the
‘Press F9 method’ but in STEADYSED1 as well.
Only by conducting measurements of the material
fluxes in-situ at very dissimilar locations, and
after repeated comparisons with the adequate
models have been made, will the previous model
concepts and the calculated fluxes ever become
verified in the future.
Acknowledgements
This is contribution No 0337 of the Research Center
Ocean Margins (RCOM) which is financed by the
Deutsche Forschungsgemeinschaft (DFG) at Bremen
University, Germany.
15.5 Problems
Problem 1
Table 15.1 shows an analysis of ocean water
made by Nordstrom et al. (1979). A condition of
oversaturation of the minerals calcite (SI = 0.76)
and dolomite (SI = 2.4) at a measured pH value
of 8.22 has been calculated for this analysis
with the model PHREEQC (see also Table 15.3).
What pH is to be expected if we calculate the
saturation state of both minerals in this ocean
water sample with PHREEQC under the condition of log pCO 2 = –3.47 ?
Problem 2
At which pCO 2 would the mineral calcite in
the ocean water sample in Table 15.1 (Nordstrom et al., 1979) be saturated if a pH value of
8.22 had been measured ? How could this pCO 2
which is markedly below the value of the
atmospheric be explained ?
Problem 3
What penetration depth of oxygen would
you expect in a sediment, if the removal of
oxygen can be described in terms of a half-life
of 5 days? Sediments have a temperature of
10°C. Their porosity is at 70 percent. For the
diffusion coefficients use Tables 3.1 and 3.2.
The bottom water shall have an oxygen concentration of 200 µmol l
-1
. Use an oxygen concentration less than 1 µmol l
-1
as “zero”.
Problem 4
A different sulfate concentration gradient is
measured above the sulfate-methane transition
zone than underneath the methane zone. How
could this be explained, despite the fact that
sulfate and methane react with each other in a
ratio of 1:1 ?